1. Field of the Invention
The present invention generally relates to a load control system for preventing an error operation of a driving unit when a signal input line of a driving circuit is short-circuited, more particularly relates to a load control system for a blower motor of a vehicle air conditioner.
2. Description of Related Art
In recent years, a motor without a commutator and a brush (hereinafter, referred to as "motor") is used as a blower motor of a vehicle air conditioner, and a rotation speed of the motor is controlled by performing a pulse-width modulation control (PWM control) relative to a driving signal of an inverter for controlling electrical power applied to an armature winding of the motor.
For example, in a vehicle air conditioner, an electrical relay 140 which is turned on when an ignition switch 114 of a vehicle engine turns on is disposed between a vehicle battery 113 and a motor driving circuit (MDC) 117 of a blower motor 110, as shown in FIG. 6. That is, electrical power is supplied from the vehicle battery 113 to the motor driving circuit 117 through the electrical relay 140.
Further, as shown in FIG. 6, an armature winding 111 of the blower motor 110 is delta-connected, and an air-conditioning control unit (A/C ECU) 115 is connected between the ignition switch 114 and the motor driving circuit 117. The air-conditioning control unit 115 calculates a target rotation speed of the blower motor 110 and outputs a duty signal Dt. The duty signal Dt output from the air-conditioning control unit 115 is input into the motor driving circuit 117 through a signal input line 116. In the conventional system, the rotation speed of the blower motor 110 is controlled by the motor driving circuit 117 to be increased in proportion to an increase of the duty signal percentage (ratio) Dt (%), as shown in FIG. 7. Therefore, when the motor driving circuit 117 of the blower motor 110 or the air-conditioning control unit 115 has a trouble and full-load operation of the blower motor 110 is performed, the ignition switch 114 is turned off by a passenger, so that the electrical relay 140 is turned off and operation of the blower motor 110 can be stopped.
However, when the electrical relay 140 is not provided for reducing cost, the motor driving circuit 117 is directly connected to the vehicle battery 113. In this case, when the signal input line 116 for inputting signal to the motor driving circuit 117 is short-circuited on a vehicle side (i.e., ground short-circuited), 100% of duty signal Dt is applied to the motor driving circuit 117. As a result, the full-load operation of the blower motor 110 is continued even when the ignition switch 114 is turned off in this case, and over-discharge of the vehicle battery 113 may be caused.